<!DOCTYPE html>
<html lang="zh-CN">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <!-- 某个元素的说明 -->

  <!-- 用法一 -->
  <label for="username-input">用户名：</label><input id="username-input" type="text">

  <!-- 用法二 -->
  <label>密码：<input type="password"></label>
</body>

</html>